The present invention generally relates to rotary couplers. The present invention more specifically relates to a capacitively coupled rotary coupler for use in a minimally invasive imaging catheter and system.
Intravascular catheters such as intravascular ultrasonic (IVUS) catheters enable imaging of internal structures in the body. In particular, coronary IVUS catheters are used in small arteries of the heart to visualize coronary artery disease. An IVUS catheter will, in general, employ at least one high frequency (20 MHz-45 MHz) ultrasonic transducer that creates pressure waves for visualization. At least one transducer is typically housed within a surrounding sheath or catheter member and mechanically rotated for 360° visualization.
The highest frequencies at which commercially available coronary imaging catheters operate are 40 MHz and 45 MHz. These high frequency probes have an axial resolution of approximately 200 microns. An axial resolution of 200 microns is insufficient to resolve structures with size features smaller than 200 microns. For example, thin-cap fibroatheromas having a thin fibrous cap of 65 microns or less in thickness cannot currently be resolved. The concern regarding thin-cap fibroatheromas is that they are prone to rupture. Plaque rupture can lead to thrombus formation and critical blockages in the coronary artery. The ability to reliably identify thin-cap fibroatheromas can aid interventional cardiologists to develop and evaluate clinical treatment strategies in order to reduce post percutaneous coronary intervention morbidity rates. Therefore, IVUS catheters and systems having improved axial resolution capable of more clearly visualizing micron sized features such as vulnerable plaques are needed in the art. The ability for such systems to operate at high transducer frequencies will be important in that effort.
One of the challenges of these minimally invasive imaging systems is coupling the stationary ultrasound transceiver (transmitter/receiver) to the mechanically rotating transducer. Rotary inductive couplers are used in commercially available IVUS systems. However, rotary inductive couplers are non-ideal for very high frequency (30 MHz-300 MHz) operation because of their relatively high series inductance. At such high frequencies, series inductance will result in an insertion loss into a transmission line of the IVUS catheter. Furthermore, the insertion loss increases with increasing ultrasound imaging frequency which degrades system performance. Rotating inductive couplers also exhibit electrical impedance that can vary with rotational position. The variation of impedance with rotational position causes output signal amplitudes to vary with angular positions and further degrades system performance. The present invention addresses these and other issues towards providing imaging catheters having improved resolution and more constant level output.